ECHINOIDEA 643 



are the smallest and have very flexible stalks and broad, blunt jaws. It 

 is said that the gemmiform kind are weapons of defence against large 

 foes, the tridactyle against small, the ophiocephalous seize small 

 animals for food, and the trifoliate destroy debris. The peristomial 

 edge of the corona is indented in each interradius by two notches, 

 where stand the ^/7/^— delicate, branched outgrowths of the body wall, 

 each containing a cavity which is continuous with the lantern coelom 



e. . aos. 

 A7i. / ,inad. 



mad.ves. 



-amp: 



Fig. 450. A diagram of a vertical section of Echinus, passing on the left 

 through a radius and on the right through an interradius. Certain structures 

 not immediately in the plane of section are shown. A, Whole section. B, The 

 region of the madreporite. amp. ampulla of tube foot; amp.' madreporic 

 ampulla; An. anus; aos. aboral sinus, with genital rachis; arc. arch ossicle 

 of jaw; aur. auricula; d.b.v. "dorsal blood vessel"; e. pigment spot in ocular 

 plate; epin. epineural canal; g.sto. genital stolon; gill, external gill; j. jaw (not 

 strictly in section) ; j.' lower part of the same ; l.coe. lantern coelom ; M. mouth ; 

 m. muscle (protractor) which pulls down the jaw and protrudes the tooth; 

 m.' muscle (retractor) which pulls back the jaw; mad. madreporite; mad.ves. 

 madreporic vesicle ; n. nerve ring ; oe. oesophagus ;^er.coe. perivisceral coelom ; 

 ra.n. radial nerve ; ra.wv. radial water vessel ; rm. rectum ; rot. rotula (the com- 

 pass which overlies this is omitted) ; sh. shell (corona) ; sip. siphon ; st. stomach ; 

 stc. stone canal; T.bd. Tiedemann's body ("Polian vesicle"); tf. tube foot; 

 tth. tooth; v.b.v. "ventral blood vessel"; wvr. water vascular ring. 



(see below). Ten little plates on the peristome around the mouth 

 carry openings for the ten short, stout, sensory buccal tube feet, the 

 proximal pair of podia in each radius. 



The mouth, which is surrounded by five strong, slightly projecting, 

 chisel-shaped, interradial teeth, leads into a relatively narrow oeso- 

 phagus, whose lower part is enclosed in a framework, known as 

 Aristotle's lantern (Figs. 450, 451), which supports the teeth. The 



